Negative index handling

[Technologicat]

Make sure unpythonic handles negative indices the same way as standard Python.

Most of the code should already be correct, but particularly, at least in the unpythonic.collections module (see _make_negidx_converter), I've accidentally modded by the length instead of just adding the length once when it gets a negative index.

(This may well be the only place that needs fixing. Probably still a good idea to grep for the % operator in all of the codebase.)

The current behavior may hide some IndexErrors in user code (e.g. trying to access lst[-8] when len(lst) == 5), and should be considered a bug.